The invention concerns a process to regulate the thickness of tubular film that is manufactured in a film blowing process, by laying the film flat and repositioned it using an oscillating take-off unit and subsequently orienting the film monoaxially in a machine direction using an orientation unit. The invention furthermore concerns equipment to implement the aforementioned process.
The film blowing process is ideal for producing plastic film that is suitable for subsequent orientation. The film is oriented monoaxially in machine direction in orientation units, which results in a reduced film thickness. Orientation is employed to improve the following film properties, for example: tensile strength, stiffness, transparency, barrier properties and/or ease of movement through the machine. This type of film is used, for example, in flexible packaging.
In the production of tubular film, film thickness profile control systems are employed that have segmented control zones. It is possible with these systems to regulate the film thickness profile such that the thickness deviations around the entire bubble circumference are as minor as possible.
In patent DE 100 47 836 A1, a process to regulate the film thickness profile in film blowing lines is described that is based especially on measurement of one or more individual layer thicknesses of a multi-layer film as the controlled variable.
Systems to regulate the film thickness in longitudinal orientation units for cast film or laminating film are also known. Regulation is much simpler in this case because the film is not reversed, meaning that a direct assignment of the individual test points in the transverse thickness profile of the longitudinally oriented film to the extrusion die including actuators and/or the control zones is always given.
DE 39 41 185 A1 discloses a process to regulate the film thickness of tubular film exiting film blowing lines with downstream axial or biaxial orientation of the inflated tubular film inside an oven, so that the final film has as few thickness deviations as possible.
During longitudinal orientation with an orientation unit, however, the film is stretched in accordance with the orientation ratio in machine direction, thus reducing the thickness of the film. At the same time, the film necks in transverse direction, which causes the width of the film to decrease. This “necking” results in the oriented film thickening slightly in a progressive manner from the center to the edges of the film, even though prior regulation in the film blowing process resulted in a thickness that was as constant as possible. This increase in thickness is particularly visible in the area around the film edges. And this in turn leads to an edge build-up on the film reel during downstream winding. With increasing reel diameter, the edges of the film web stretch more and more, and this presents a serious disadvantage for downstream processing steps such as printing or lamination.
It is possible to reduce the necking and thus the edge build-up on the film reel by taking measures such as providing the smallest possible orientation gap, a suitable roll coating, mechanical or electrostatic fixing of the film edges, optimized temperature management or a suitable selection of the polymer materials. The reduction thus achieved by such steps, however, is inadequate for many downstream processing steps. Only suitable trimming of the film edges will result in the remaining film web, having a sufficiently low deviation of the thickness profile that is required for subsequent film winding and downstream processing. Trimming, however, always means that a significant amount of the film is wasted and is associated with a loss of approx. 200 mm on each side, independent of the film width.
Accordingly, there is a need for improvements in generating films of uniform thickness and these are now provided by the present invention.